Anomalous Positive Ion Formation in Negative Ion Collisions with

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Anomalous Positive Ion Formation in Negative Ion Collisions with HOPG Pinyang Liu, Lin Chen, Shunli Qiu, Feifei Xiong, Haoyu Jiang, Jianjie Lu, Yuefeng Liu, Guopeng Li, Yiran Liu, Fei Ren, Yunqing Xiao, Lei Gao, Bin Ding, yanling guo, and Ximeng Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03680 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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The Journal of Physical Chemistry

Anomalous Positive Ion Formation in Negative Ion Collisions with HOPG Pinyang Liu†,‡, Lin Chen*,†,‡, Shunli Qiu†,‡, Feifei Xiong†,‡, Haoyu Jiang†, Jianjie Lu†,‡, Yuefeng Liu†,‡, Guopeng Li†,‡, Yiran Liu†,‡, Fei Ren†,‡, Yunqing Xiao†,‡, Lei Gao†,‡, Bin Ding†,‡, Yanling Guo†,‡, and Ximeng Chen*,†,‡ †

School of Nuclear Science and Technology, Lanzhou University, 730000, China Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, 730000, China ‡

Abstract: Charge transfer on graphite, a typical substrate and one of the fusion first wall materials, is of great importance in plasma-wall interaction, thin-film growth and surface catalysis. We present an experimental study of 8.5-22.5 keV-energy carbon, oxygen and fluorine negative ions scattering from a highly oriented pyrolytic graphite (HOPG) surface at a scattering angle of 8°. It is found that the positive ion fraction decreases monotonically with the increase of both incident velocity and angle. In particular, these dependences are very different from those presented in previous studies. A molecular dynamics simulation reveals that, around the critical condition for planar surface channeling, a number of projectiles may penetrate into the subsurface and become energetic atoms when they emerge from the surface. Hence, an exponential scaling related to the penetration probability has been proposed to well describe the velocity and angle dependences of positive ion fractions.

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1. INTRODUCTION Carbon has been widely used for its various allotropes and abundant molecular configurations. One of its well-defined forms, graphite, can be used as a typical substrate for adsorption,1 growth,2 morphology,3 photochemistry and photocatalysis.4 It is also attractive in tokamaks devices, to be used as first wall and divertor plate.5 In recent years, the ferromagnetism in HOPG has been observed. 6-9 Charge transfer on HOPG is one of the important topics of relevant areas, due to not only its basic interests in physics and surface chemistry but also its importance in the quantification of surface analytical techniques, such as low-energy ion spectroscopy (LEIS)10 and secondary ion mass spectroscopy (SIMS).11 It is also involved in other application fields, such as plasma-wall interaction in fusion research12 and thin-film growth technologies.13 In particular, negative (positive) ion formation involved in charge transfer processes, can be used for particle detection in interplanetary and interstellar space because of a low detection efficiency of neutrals,14 and has been proposed to be used in space propulsion thrusters, 15,16 and also has been chosen to produce a large neutral beam current for the neutral beam injector (NBI) system in future fusion technology, such as the International Thermonuclear Experimental Reactor (ITER).17 At present, the most direct way to probe positive or negative ions is the measurement of charge state distribution of scattered particles using the ion-surface scattering technique.18-21 In this work, we experimentally study the charge transfer process, i.e., positive ion formation, for energetic negative ions scattering on HOPG. Most studies of charge transfer on HOPG up to now have been confined to proton projectiles. H/HOPG is considered as a model system since hydrogen is the simplest projectile.22-26 Positive and/or negative hydrogen ions on HOPG surface have attracted more attention in the past. Goldberg’s group reported that H+ fraction from HOPG shows a monotonic increase with incident energy from 7% to 20% with primary energy in the 2-8-keV range in large angle scattering and backscattering, and that H- fraction remains almost constant.22,23 Moreover, H+ fraction shows an increase with increasing exit angle in the 7-12% range, while H- fraction shows the opposite tendency23 for 4 keV H+ scattering at large angles. H- fraction in grazing scattering from HOPG has been reported by Esaulov’s group where the H- fraction increases with exit angle at 4 keV incident energies,24 but the positive-ion fraction is not available. These studies make considerable effort to understand the high H- fraction which surprisingly conflicts with our understanding because of the low electron affinity of H- (0.75 eV) as compared to the work function of HOPG (about 4.6 eV). However, the positive-ion formation has not been the subject of similar attention as compared to a lot of research for negative-ion formation. One of the possible reasons is that the incomplete memory loss of the initial charge state may interrupt the final positive-ion formation during positive-ion surface scattering.27 But it can be avoided by using neutral and negative ion beams. So far, direct experimental studies using negative-ion beams have been much scarcer because the negative-ion source is needed. Hence, it motivated our present work. To date, the interest of charge transfer of multi-electron projectiles scattering on 2


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HOPG has grown strongly since such projectiles can be used to examine many-body theory.28-34 However, in some tens of keV energy range, few experimental investigations have been conducted.34 Therefore, much effort should be devoted to microscopic understanding of underlying scenario of positive-ion production on graphite surfaces. In this work we have adopted carbon, oxygen and fluorine negative ions and a scattering angle of 8° that is close to the critical condition for planar surface channeling. It is found that both the incident velocity and angle dependences of positive-ion fractions differ from our previous studies.35,36 In particular, positive-ion fractions decrease with increasing incident energy. This anomalous dependence has contrasted with a lot of previous results in the low energy range.22,23,36-42 A molecular dynamics simulation of ion trajectories has been presented and significantly improved our knowledge of positive-ion production on HOPG. The organization of this paper is as follows. In Section 2 we describe our experimental apparatus. After a presentation of the experimental method, we will present the dependences of ion fractions with both incident angle and energy in Section 3. The full results are discussed in Section 4. 2. EXPERIMENT The experiments of ion surface scattering were performed in the apparatus as shown in Figure 1. The C- (O-, F-) ions were produced in a cesium sputter negative-ion source and then deflected by a 45° bending magnet. The extracted ion beam passed through a pair of electrostatic plates placed between two slits. The negative component separated from neutrals passed through the third collimators 30.5 cm downstream of the second slit, before entering an ultrahigh-vacuum (UHV) chamber, with a typical pressure of better than 3×10-7 Pa. Three collimators guaranteed the angular divergence of the primary beam less than 0.1° (full width at half maximum (FWHM)) for all measurements via reducing the beam size. For surface scattering with a scattering angle of 8°, the incident angle was varied from 1° to 7° measured with respect to the surface plane. To avoid particle scattering on the tube wall, the scattered beam from the surface passed through the two post-target separated slits with 1×2 mm2 apertures. The charge states of scattered particles were then analyzed by a parallel-plate electrostatic deflector. A one-dimensional position sensitive microchannel plate (PSMCP) detector was located 60 cm downstream of the deflector at the end of the tube. The detector essentially consisted of a resistive anode placed behind two MCPs mounted in a chevron configuration. The detector was well fixed at appropriate bias voltage where the measured count rate becomes independent of pulse height. The detector efficiency increased with impact energy and was assumed to be identical for scattered particles with different charge states.43-45 A multi-parameter acquisition system (MPA-3) (FAST ComTec) was used for data collection. We appropriately choose the bias voltage of the deflector to ensure that all scattered particles are well collected by the detector. The position spectrum of scattered particles is shown in Figure 2. The biggest peak corresponds to the scattered 3



neutrals, and the positive and negative ions are symmetrically located on its both sides. The positive ion fraction is defined as: Φ + = N (+) / N (total ) , where N(+) is the number of scattered positive ions corresponding to area of the positive ion peak, and N(total) is the total number of scattered particles corresponding to the sum of areas of three peaks. Due to signals recorded in coincidence mode for the PSMCP detector, the signal-to-noise ratio is high, which guarantees the detection accuracy. The data are reproduced via a series of measurements done in different days. The experimental error in the fraction is mainly determined by counting statistics, and is less than 10%.

Figure 1. Schematic diagram of the experimental setup for ion scattering measurements.

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Figure 2. The position distribution of scattered particles for 22.5-keV negative fluorine ions in specular scattering on a HOPG surface. The HOPG sample was purchased from MaTeck and manufacturer quoted a mosaicity of less than 0.8°. To obtain an atomically flat surface, the layered structure of the graphite surface simplified the preparation procedure by removing several top 4


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layers with adhesive tape prior to insertion to the UHV chamber. Figure 3 displays three-dimensional atomic force microscopy (AFM) image over 1×1 µm2 at the prepared HOPG surface. It is observed that about 60% of the total surface is relatively flat. Atomic size steps are shown in atomically flat areas. These steps are about 3 Å in height, which corresponds to a space between the layers in graphite and is consistent with previous work.46 The HOPG sample was mounted on a sample holder supported by a 4-dimensional precision manipulator and was heated by electron bombardment. The extremely low adsorption coefficient of compounds and absence of oxidation layer on the surface ensure cleanliness. During the whole experiment, the sample was prepared by cycles of annealing at about 773 K for 30 minutes to prevent the contamination. Measurements were made for the incident energies varied from 8.5 up to 22.5 keV.

Figure 3. Surface morphology of clean HOPG obtained by AFM detected under force modulation mode. Typical technical data, Thickness: 3µm/length: 225µm/width: 28µm; resonance frequency: 75 kHz; force constant: 2.8 N/m and coating: none. The surface is relatively flat and atomic size steps or corrugations are shown.

3. RESULTS In Figure 4 we show positive-ion fractions as a function of incident velocity for carbon, oxygen and fluorine negative ions scattering on a HOPG surface at a scattering angle of 8°. In Figure 4(a), the C+ fraction is large and up to about 26%. It decreases sharply with increasing incident velocity. While in Figure 4(b) O+ and F+ fractions are smaller than C+, and both of them also decrease with the increase of velocity with small fluctuations which is attributed to experimental errors. In addition, F+ fractions are the smallest as shown in the figure. In Figure 4 we observe a quite surprising result that the positive ion fraction decreases with increasing incident velocity. This is substantially different from what happens on metallic surfaces37-39 where scattered Ne+, Ar+ and O+ fractions increase monotonically with primary kinetic energy in the range of several keV energies. He+ fraction of grazing scattering off Al (111) pronounced increases with energy after 5



reaching a kinematic threshold of several keV.40,41 Moreover, for semiconductor surfaces, F+ increases linearly with energy in the range of 8.5-22.5 keV, 36 and Ne+ grows rapidly from 2 to 10 keV energies and then becomes almost constant for higher energies.42 For HOPG surface, Goldberg’s group reported that H+ fraction also shows a monotonic increase with incident energy from 7% to 20% with primary energy in the 2-8-keV range in large scattering angles of 45° and 135°.22,23 To the best of our knowledge, such an anomalous dependence on incident velocity in our case has never been observed.

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Figure 4. (color online) (a) C+ fractions (b) O+ and F+ fractions from HOPG as a function of incident velocity for specular scattering. The black solid, red dashed and blue dash dotted curves are the calculated results. In Figure 5, we show positive-ion fractions of scattered carbon, oxygen and fluorine ions as a function of incident angle. In general, C+ fraction is greater than O+ fraction and O+ fraction is slightly larger than that of F+ for the same incident angle and energy. All these positive ion fractions decrease monotonically with increasing incident angle. It is also observed that positive ion fractions decrease more steeply for incident angles below 4°. For a given incident angle, the larger the incident energy, the lower the positive-ion fraction. Similar experimental results have been reported by Goldberg’s group,23 in which + H fraction decreases with increasing incident angle. The descending rate of the H+ fraction decreases with increasing incident angle for 4 keV H+ scattering on HOPG at a scattering angle of 45°. It is also found that the descending rate of the Ne+ fraction scattering from an amorphous silicon surface gradually decreases when the incident angle increases.42 However, for F- and Cl- scattering on Mg and Ag,47 F+ and Cl+ fractions first decrease steeply, then keep almost unchanged and even slightly increase 6


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with incident angles at a small incident energy of 1 keV, while for large projectile energies, the descending rate increases with increasing incident angle. For Fscattering on Si,36 the F+ fraction decreases linearly with increasing incident angle for 9.5 keV, and for energies of 13.5 and 21.5 keV, F+ fractions trend to be unchanged at small incident angles and then decrease monotonously at large incident angles. In principle, the positive ion fraction in these studies at large energies36,47 can be understood in terms of the exponential dependence of the inverse of exit angle for a given energy.48,49 It is associated with the survival probability of positive ions during Auger and/or resonant neutralization along the outgoing trajectory proposed by Hagstrum.49 As we can see, the first two mentioned studies,23,42 as well as ours, are different from other studies.36,47 The descending rate of positive-ion fraction as a function of incident angle cannot be well explained by the exponential scaling as mentioned above. As a consequence, the reason for new findings about the dependence of positive-ion fraction on incident velocity and angle will be discussed in details in the next section.

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(a) C+ 22.5keV 22.5keV 0.06

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Figure 5. (color online) (a) C+ (b) O+ and (c) F+ fractions as a function of incident angle at different incident energies as indicated. The black solid line is calculated C+ fraction for 22.5 keV energies. The magenta dash dotted, navy dotted and cyan dashed lines correspond to O+ fraction for 22.5 keV, 18.5 keV and 14.5 keV energies respectively. The pink short doted, blue short dashed and green dash dot dotted lines correspond to F+ fraction for 22.5 keV, 18.5 keV and 14.5 keV respectively. 4. DISCUSSION 4.1. Projectile trajectories. For grazing surface scattering, both axial and planar channelings play a role. Axial channeling takes place between strings of lattice atoms and depends on the crystal structure and its orientation relative to the incident beam. Planar channeling occurs in front of the topmost surface layer or between pair of 7



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planes in the bulk.50 For planar channeling, a critical angle is roughly described by Φ crit = ( 2πans Z1Z 2 / E0 )1/ 2 , where the screening length a = 0.8854( Z11/ 2 + Z 21/ 2 ) −2 / 3 , ns is the number of surface atoms per unit area, Z1, Z2 are projectile and target atomic numbers, and E0 is the projectile energy. The planar surface channeling occurs below the critical angle and the projectiles will be scattered directly by the 1st-layer atoms due to a collective potential effects of target surface atoms, i.e. the specular reflection. Therefore, for charge transfer of keV-ion/atom surface scattering under a small grazing angle of incidence, i.e., 1°, the scattering event occurs above the surface because the incident angle is smaller than the related critical angle. It is easy to understand that projectiles have a large probability to penetrate into the surface when the incident angle is larger than the critical angle.51-54 For scattering on a HOPG (0001) surface [ns=3.845×1015 atom/cm2 55], we find the critical angle is from about 5.18° to 8.88° for C- energies from 22.5 keV to 8.5 keV. For O- with the same energies it is about 5.83° to 10.00° and for F- ions it is about 6.12° to 10.49°. The critical angles are covered in the incident angle range used in our experiments. As we know, HOPG has a relatively loose atom configuration of hexagonal lattice structure. Moreover, for ion surface scattering, its small atomic number of Z2=6 corresponds to a smaller coulomb repulsion as compared to other high Z2 materials. Thus, in our case, it is likely that a part of projectile trajectories may come from the subsurface and can be detected. It is also further supported by the trajectory simulation carried out with the Kalypso software package.56 Briefly, Kalypso is a molecular dynamics software for simulation of atomic collisions in solid. The target used in this work consists of four atomic layers, comprising approximately 13000 atoms. The target was very long in the direction of the ion path to allow continuous collisions. Thermal vibrations of the target corresponding to room temperature (T=300K) were considered in the simulation. The projectile impact points lied at a hexagonal grid in a representative symmetry area of the surface atoms. The F-C interaction potential was represented by uncorrected Ziegler–Biersack–Littmark (ZBL) universal screening function potential.57 To speed up the ion scattering spectroscopy (ISS) simulation, we did not model an attractive potential between the projectile and the target, or between target atoms.56 In order to obtain enough trajectory number, we increased the acceptance angle of the detector appropriately. The simulations were terminated after 200 fs, or the projectiles located >3 Å above the surface or v0 ) v sin α − v0

(1)

where A, λ, v0 are free parameters. v0 is regarded as a perpendicular threshold velocity. The projectile cannot penetrate into the surface when its incident perpendicular velocity is below v0. v is the incident velocity, and α is the incident angle with respect to the surface plane. In order to estimate the penetration probability δ, we performed a series of trajectory simulations for F- particles scattered from HOPG with different incident angles and incident energies. In the case of the incident energy dependence at specular scattering (4°/4°), five different incident energies were used: 8.5, 12.5, 16.5, 18.5 and 22.5 keV. The scattered particles were collected with an altitudinal angle of 4±1° and 4±2°, and with a plane angle width of ±1° (in polar plane). In the case of the incident angle dependence for a given incident energy of 22.5 keV, five different angles of 2°, 3°, 4°, 5° and 6° were used. The scattered particles were collected with an altitudinal angle width of ±2°, and with a plane angle width of ±1° (in polar plane).

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0

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x (angstrom) Figure 6. Trajectories of 22.5 keV F- projectiles impinging on the HOPG surface with an incident angle of 4°. (a) side view corresponds to the projection onto a plane perpendicular to the surface (XZ plane). (b) top view to the projection on the surface plane (XY plane); Typical trajectories were emphasized by thick lines with different color. Squares represent C atoms. It is noted that, for the incident energy dependence, no penetration trajectory is found for 8.5 and 12.5 keV energies in both altitudinal angle sets. It indicates that the critical angle for planar surface channeling is large for low incident energies, which is generally consistent with the relationship described with the formula of critical angle 10


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mentioned above. For the incident angle dependence, no penetration trajectory is found for incident angle of 2°, while for incident angle of 6° only dozens of projectiles are recorded and most of them are escaped from the edge of the built target. In practice, we try to further increase the size of the target, however there are still trajectories out of the boundary of the target. In the same time, the computation time makes us so unbearable. We display the penetration probability δ as a function of the incident perpendicular velocity in Figure 7 for different energies and angles. The simulation data are well described by eq 1 with the solid red line with A = 0.3243, λ = 0.0023, and v0 = 0.01066. The penetration probability δ increases slowly with the increase of velocity and appears to have a saturated trend. With the increase of the perpendicular velocity, the simulated trajectories become more complex and the penetration depth is deeper.

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perfect surface, energy dependence, α =±10 perfect surface, energy dependence, α =±20 perfect surface, angle dependence, α =±20° defect surface, angle dependence, α =±20 δ in Eq.(1) of perfect surface δ in Eq.(1) of defect surface δ1 for carbon δ2 for oxygen and fluorine

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Figure 7. The penetration probability δ for F- scattering on HOPG as a function of the incident perpendicular velocity: (■) for the perfect surface, incident energy dependence, detector acceptance angle of ±1°, and (▲) for detector acceptance angle of ± 2°; (●) for the perfect surface, incident angle dependence, detector acceptance angle of ±2°; (▼) for defect surface, incident angle dependence, detector acceptance angle of ±2°. The red and green solid lines (δ) are the fitting results for the perfect surface and defect surface respectively. The blue dash dotted line (δ1) is used for the calculation of carbon positive-ion fraction, and the black dashed line (δ2) is used for the calculation of oxygen and fluorine positive-ion fractions. In practice, another factor which should be taken into account is the quality of 11



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the surface, which is a key component in any surface scattering experiment, particularly for a grazing-angle geometry that is vulnerable to surface defects and contamination. As shown in Figure 3, there are apparent steps and corrugations in HOPG. Thus the surface topography could to some extent influence the trajectory of projectiles. Projectiles can enter into sublayers from the step edge, and a larger penetration probability would be expected. The topography of HOPG is analyzed by scanning tunneling microscope (STM)3 and also shows clearly terraces, which means that those defects cannot be fully ruled out by the current technical preparation of target. From the point of view of the surface roughness, we randomly add some steps for a defect surface in the simulation to compare with the perfect surface. For the defect surface, the incident angle dependences of 2°, 4°, and 6° at 22.5 keV energies were simulated and collected with an altitudinal angle width of ±2°. The related results are also shown in Figure 7, and the simulation data are well described by eq 1 with A = 1, λ = 0.0006432, and v0 = 0.007043. It clearly shows a larger penetration probability and a smaller perpendicular velocity threshold for the defect surface.

4.2. Positive ion fractions. For negative ion scattering on HOPG, the complete neutralization occurs in the incoming path and the negative projectiles first become atoms via resonant electron loss to the conduction band of HOPG prior to form positive ions. The positive ions must be produced through inelastic collisions with surface atoms. The inelastic collisions include both the ionization and electron excitation processes. To produce positive ions, direct single ionization process takes place via Coulomb repulsion well described by the binary encounter approximation (BEA).58-60 The molecular orbital (MO) promotion model has been successfully employed to describe the electronic excitation process in ion surface collisions.61-71 We briefly point out the possible electron excitation for C-C system are caused by 3dσ MO promotion that is associated with the 2p level of C.62 Since the electron energy level ordering of C is identical with O and F with respect to C target, similar electron promotion for O and F should also occur. The MO correlation diagram of O-C and F-C constructed following the Barat-Lichten rules61 is shown in Figure 8. Electron promotion occurs only if a critical internuclear distance (Rmin) is reached. An adiabatic MO correlation diagram for selected orbitals of C-C has been calculated by the Hartree-Fock method and the computer code Gaussian.62 From the correlation diagram, we find that the promotion along the 3dσ MO occurs near Rmin≈1.2 a.u.. In our case for C-, O- and F- scattering at a scattering angle of 8°, the distances of closest approach calculated by Thomas-Fermi-Moliere potential with the Firsov screening length63 for the maximum energy of 22.5 keV are about 0.21 a.u. for C, 0.26 a.u. for O and 0.28 a.u. for F, respectively. For the minimum energy of 8.5 keV, the corresponding distances are 0.40, 0.47 and 0.50 a.u., respectively. Therefore, in our case electron promotion occurs. According to the MO correlation diagram in Figure 8, 3dσ MO of the quasi-molecule can be highly promoted in energy so as to cross other MOs, and then excitation via the radial coupling at the 3dσ-3sσ, 3pπ crossings and the rotational coupling at 3dσ-3dπ, 3dπ-3dδ crossings occurs where one or two electrons of the 12


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incident projectile may be involved. In general, at these crossings, the projectile’s electron can transfer to its Rydberg states, forming an excited state. 61,63-69 On the other hand, the projectile’s electron can also transfer to target atoms, forming a positive ion which may survive neutralization. As we know, the distance of merging of the promoted diabatic level to the continuum for C-C system is about 0.7 a.u.. 62 The distance of closest approach in our case is smaller than this value as mentioned above. Therefore, once the promotion of 2p level of the projectile is strong, the promoted electron may go to the surface during a close collision via resonant ionization with unoccupied states of the surface.70 As the ionized particle recedes from the surface, electron capture may occur and excited states will be populated again.71 Thus, the singly excited state and/or the doubly excited state can be formed in this process via one and/or two electron capture. On the other hand, the ionized particle may survive during the collision process. Such a physical scenario can be also applied to the case of O and F. For a single-electron promotion process, singly excited state is formed and then immediately become positive ions via resonant ionization with unoccupied states of the surface.63-65 For a double-electron promotion process,66-69 C+ (O+, F+) ions may be produced through the following scheme,

C − (O − , F − ) ionization  → C 0 (O 0 , F 0 ) MO → C ** (3s 2 )(O ** (2 p 2 3s 2 ), F ** (2 p 3 3s 2 )) AI → C + (O + , F + )

where in approaching the surface, C- (O-, F-) is ionized through resonant ionization (resonant electron loss) to form C0 (O0, F0) which by double-electron promotion of C 2p3 (O 2p4, F 2p5) subsequently forms C** (O**, F**) in hard collisions with surface atoms. These doubly excited atoms then decay far from the surface via autoionization (AI) to give C+ (O+, F+) ions.

Figure 8. Qualitative MO correlation diagrams for the O-C and F-C collision systems. 13



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a stands for O and F atoms. In particular, another excitation process should be mentioned here. In collisions of open shell atoms (groups V, VI, and VII), the ground configuration of its positive ion corresponds to several states, e.g., 4S⁰, 2D⁰ and 2P0 for O+, 3P, 1D and 1S for F+. The upper members of the Rydberg series converging to the higher ionization limits lie above the first (or second) ionization limits and are therefore autoionizing.72-75 Such excited atoms could become positive ions via autoionizing after receding from the surface, and may become atoms in ground state by Auger de-excitation processes. Thus, in the case of F scattering, the 2pπ4 core may dissociate into a mixture of 1D and 1S states, at high energies a statistical population (2:1) should be reached. The excited states for higher configurations 1D nl and 1S nl series thus are formed. For O, the 2pπ3 core may dissociate into 2D and 2P O+ states and hence one may expect the population of states of the 2D nl and 2P nl series. For C, the ground configuration of positive ion corresponds to only one state (2P), such excited states are not available. Besides, another possible mechanism for positive-ion formation is kinematically induced Auger ionization. It is a kinematic process occurring in small scattering angles. Auger ionization is a two-electron process, an electron can go from an occupied state into an empty state of the surface with energy transferred to lift an atomic electron to an unoccupied state of the surface.41,48,76 Energy conservation requires kinetic energy from the projectile to excite the electronic system and therefore Auger ionization is only possible to occur above a threshold kinetic energy of the projectile, i.e. a threshold velocity vth is deduced,

vth = 3v F (1 − 1 − ( Ea − W ) / 9ε F ) , (2)

EI 1 − )a.u. is upward 27.2 4 z shifted from the first ionization energy EI of the projectile by image potential effects at a distance z measured from the image plane. With work function W=4.6 eV and where ε F is the Fermi energy of the surface, and Ea ( z ) = (

ε F = 22 eV45 for HOPG, from eq 2, we deduce that for CI 2P2 3P0 (EI=11. 26 eV), vth=0.065 a.u. or Eth=1.3 keV; for OI 2P4 3P3/2 (EI=13.62 eV), vth=0.088 a.u or Eth=3.1 keV and for FI 2P5 2P3/2 (EI=17.42 eV) vth=0.126 a.u or Eth=7.5 keV at infinity. This threshold velocity decreases with the decrease of ion surface distance and can be satisfied by the minimum incident velocity of 0.204 a.u. (equivalent to projectile energy of 12.5 keV) for C, 0.177 a.u. (12.5 keV) for O and 0.134 a.u. (8.5 keV) for F. The calculation of Auger ionization rate is a very challenging task at present for the theoretical difficulty in describing the involved four different electronic states and the long range of the Coulomb electron-electron interaction.76 There is only the qualitative explanation41 for the scattering of He atoms from Al (111) under grazing angles of incidence. It illustrates that Auger ionization rate increases with increasing velocity and decreases with increasing ion surface distance, so that a remarkable effect of Auger ionization can only be seen near the surface. Based on the discussion of electron promotion and Auger ionization, in our case 14


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the initial formation of positive ions may be produced. However, the production of positive ions involving these processes is more complicated and the relative weight of these processes is not available at present. Our setup is not yet equipped with a time-of-flight spectroscopy system or an electron energy spectroscopy system that would allow us to detailedly investigate the electron excitation process. Therefore, the clarification of these electronic processes has not yet been achieved so far. In this respect, we cannot give a quantitative description, only assume a constant ionization probability Pi. As discussed above, some projectiles penetrate into the surface and are surrounded by many bulk atoms. A series of stochastic quasi-binary encounters of projectiles with target atoms will occur (see Figure 6). Thus the corresponding projectiles experience a large neutralization probability in collisions with target atoms below the surface before they are detected by the detector. For simplification, we reasonably assume that these projectiles are fully neutralized. This assumption could make sense when further considering the neutralization near the surface. As we know, the transition rate of neutralization, depending on the coupling between projectiles and surfaces, shows an exponential increase with the decrease of ion-surface distance. So projectiles have a large opportunity to capture electron near the surface. Second, although Auger neutralization is a two-electron process, it may be dominant for ion surface distances less than 2 a.u.. The smaller the distance, the larger the neutralization rate. Third, although the energy loss is ignored for a lot of scattering research, in practice the deeper the penetration of the projectile, the larger the energy loss, and the longer time they will spend near the surface. Even if a small fraction of positive ions is formed under the surface, they still have a large opportunity to be neutralized after emerging from the surface. On the other hand, the rest projectiles that stay above the surface are related to the production of positive ions in the vicinity of the surface. In general, the initially formed positive ions can be re-neutralized by resonant neutralization and/or Auger neutralization on the outgoing trajectory, which can be simply given by,

τ dP + ( z ) = − 0 e −αz P + ( z ) , (3) dz v sin β where the electron transition rate is given by τ = τ 0 e −αz , and β is the exit angle. The nuclear energy loss is negligible here because of the glancing incidence scattering,24, 68 so that the exit velocity is equal to the incident velocity. From eq 3, we obtain the final survival probability P + (∞) = P + ( z0 ) × exp(−

τ e −αz0 vc ) , vc = 0 is the so called v sin β α

characteristic velocity proportional to the neutralization rate. P+(z0) represents the initial positive-ion probability at z0, z0 is the starting point of the effective position of the outgoing trajectory. According to the discussion on projectile trajectory and positive-ion production, we can simply obtain P + ( z0 ) = (1 − δ ) × Pi . As a consequence, 15



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the final positive ion fraction can be expressed in terms of the production probability multiplied by the survival probability,

P + (∞) = (1 − δ ) × Pi × exp(−

vc ) , (4) v sin β

We proposed a perpendicular velocity dependent penetration probability in eq 1 which fits well to the simulation data for both the perfect and defect surfaces. Although we have already obtained parameter values of fluorine ions from two different surfaces by simulation, the existence of surface defects and their uncertainty make us not use the related fitting values directly for a real surface used in the experiment here. Considering that a defect surface is mainly related to the change of parameters rather than the incident perpendicular velocity dependence as shown in Figure 7, we regard A, λ and v0 as free parameters, and obtain − 0.0092 − 0.006 δ1 = 1× exp( ) for C and δ 2 = 0.69 × exp( ) for both F v sin α − 0.0001 v sin α − 0.001 and O particles since they have nearly the same atomic number. We also display the fitting positive ion fractions as a function of incident angle and velocity in Figures 4 and 5, which are described by eq 4 with

PC+ (∞) = (1 − δ1 ) × 0.5 × exp(−

0.0001 0.0001 ) , PO+ (∞) = (1 − δ 2 ) × 0.165 × exp(− ) v sin β v sin β

PF+ (∞) = (1 − δ 2 ) × 0.153 × exp(−

0.0001 ) v sin β

for

carbon,

oxygen

and

and

fluorine,

respectively. Apparently, O+ and F+ are well described by eq 4, while for C+ the general tendency is reproduced, but slightly large deviations are shown especially for small incident angles. The ionization probability Pi of 0.165 for oxygen ions is larger than 0.153 for fluorine ions. It indicates that it is easier to produce O+ ions and agrees with the fact that oxygen has a smaller first ionization energy. The reason for the largest ionization probability of 0.5 for carbon is not only due to its small first ionization energy, but also to the special C-C symmetric collision system, which leads to a large electron promotion probability.61 The characteristic velocity vc is rather small and about one or two orders of magnitudes smaller than λ for these three collision systems. It suggests that the resonant and/or Auger neutralization have minor effects on the outgoing trajectory. It may be attributed to the production of double excited-state atoms. The lifetime of double excited state is likely to be longer than the collision time [the Ne** lifetime is >10-14 s,77 and our collision time is at femtosecond time scale], doubly excited atoms can leave the surface intactly and subsequently autoionize to positive ions far from the surface where re-neutralization is unlikely.66 When ignoring the penetration effect of the trajectory, we obtain an expression for positive ion fraction Pi × exp(−

vc ) for all projectiles. It represents that the v sin β 16


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positive ion fraction is expressed in terms of the production probability at a close collision multiplied by the survival probability on the outgoing trajectory. It clearly shows a monotonous increase of the positive-ion fraction with increasing exit velocity. Moreover, the increase is faster at small exit angles, and starts to slow down at high exit angles. It is consistent with many previous observations. 36-42,47 However, this trend is not consistent with our experimental data as shown in Figures 4 and 5. Moreover, as discussed above, the survival probability related to exp(−

vc ) is v sin β

close to 1 due to the negligible vc . Now let us return to the penetration trajectory effect. The production probability is (1 − δ ) × Pi where the ionization probability Pi is a constant. The final positive ion fraction can be expressed in terms of the production probability multiplied by the survival probability, as shown in eq 4. As we know, the survival probability is close to 1, so the change of the final positive ion fraction is mainly determined by the production probability (1 − δ ) × Pi . The factor (1-δ) decreases exponentially with the increase of incident perpendicular velocity when a threshold velocity is reached, and thus it reverses the trend of the conventional velocity dependence, resulting in extraordinary decrease of positive ion fraction with increasing velocity as shown in Figure 4. It also changes the dependence of angle, resulting in the different descending rate as shown in Figure 5. Carbon has a larger penetration probability δ as compared to oxygen and fluorine (see Figure 7), which agrees with the fact that carbon has a smaller coulomb repulsion with surface atoms, smaller size of shadow cone and smaller critical angle for planar surface channeling. Moreover, the change of carbon penetration probability is faster. As discussed above, the calculated positive ion fraction mainly depends on the penetration probability, so in Figures 4 and 5 C+ fractions decrease faster with increasing incident velocity and angle as compared to O+ and F+. In Figure 5(a), the discrepancy between calculation and experiment is observed and may be attributed to the constant ionization probability Pi =0.5 used in this work. In general, ionization probability may change with the type of collision processes and with energy. The ionization probability Pi at close collisions has been calculated by Rabalais et.al. 37 based on the Fano-Lichten 78 mechanism, from which we find that the ionization probability increases with decreasing the distance of closest approach. In electron promotion model, the ionization probability during level crossing in a close collision is estimated by the Landau-Zener formula:63,79 Pi = 2 P (1 − P ), P = exp( −vc / vr ) where P is the probability for electrons to survive in the same diabatic state after crossing and vr is the relative velocity of the particles at the crossing distance. From this formula, we find that the velocity dependence is relatively complicated. In addition, surface roughness80 may also influence Pi. Therefore the calculation of Pi is a very challenging task at present and is beyond the 17



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scope of this work. For C+ fractions at small incident angles, the ionization probability may be smaller than 0.5 which makes a large discrepancy. In this respect, the ionization probability seems to present the angle dependence. As the incident angle decreases, the distance of closest approach increases, and thus the ionization probability is expected to decrease.

5. CONCLUSION In conclusion, we have reported experimental results of positive ion fractions of C , O-, F- impinging on a HOPG surface in the 8.5-22.5 keV energy range performed at a scattering angle of 8°. We observed that positive-ion fractions decrease with the increase of incident velocity and angle, which is very different from that presented in previous studies. These anomalous dependences can be well interpreted by the effect of projectile trajectory where a lot of projectiles penetrate into the subsurface and suffer from efficient neutralization. In addition, the initial formation of positive ions above the surface is mainly discussed in terms of single and/or double electron promotion and kinematically induced Auger ionization. However, the clarification of these electronic processes is not a trivial task. In this respect, we cannot give a quantitative description, and only assume a constant ionization probability. From the experimental data, we find that the resonant and/or Auger neutralization on the outgoing trajectory of positive ions seem to be negligible, which may be attributed to the formation of doubly excited atoms with long lifetime near the surface. A measurement of the electron energy spectrum may help to confirm this mechanism, which is in progress. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: 15002672420. *E-mail: [email protected]. Phone: 13519619060. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos.11405078 and 11474140), the Fundamental Research Funds for the Central Universities (Grant No. lzujbky-2014-169 and lzujbky-2015-244), the Project sponsored by SRF for ROCS, SEM, and National Students' innovation and entrepreneurship training program (Grant Nos. 201410730069 and 201510730078).

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Toc Graphic 0.34

0.12 +

+

C

0.32

Ion trajectory

O + O

C+

0.30


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+

F + F

0.28

0.11

0.26 0.10 0.24 0.22 0.09

0.20 0.18 0.16

0.08 0.20

0.22

0.24

0.26

0.28 0.12

0.16

0.20

0.24

Incident velocity (a.u.)

C atoms 1st layer

Penetration probability

δ = A × exp( 2nd layer

−λ ) v sin α − v 0

24


Anomalous Positive Ion Formation in Negative Ion Collisions with

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